Method for refining concentrate containing precious metals

ABSTRACT

The invention relates to a method for refining precious metal concentrate, and according to said method, at least the precious metal concentrate ( 9 ), reaction gas ( 10 ), flux ( 11 ) and flue dust ( 12 ) to be treated are fed together into the reacton shaft ( 3 ) of a suspension smelting furnace ( 1 ); in the suspension smelting furnace, there are created separate phases, matte ( 8 ) and slag ( 7 ); and the slag created in the suspension smelting furnace is conducted into an electric furnace ( 2 ), so that there is created metallicized matte ( 14 ) and waste slag ( 13 ). Thereafter the matte ( 8 ) from the suspension smelting furnace is conducted to hydrometallurgical treatment ( 15 ), and the slag conducted into the electric furnace is treated together with a reducing agent and possibly with a material lowering the melting point or improving fluidity, and the obtained metallicized matte ( 14 ) is conducted either to hydrometallurgical treatment ( 16 ) or back into the suspension smelting furnace in ( 1 ).

The invention relates to a method for refining precious metal concentrate defined in the preamble of claim 1. In the method according to the invention, there is used a supension smelting furnace; the matte created in said furnace is conducted into hydrometallurgical treatment, and the slag is reduced in an electric furnace. The metallicized matte created in the electric furnace is either fed back into the suspension smelting furnace or to hydrometallurgical treatment together or separately with the matte from the suspension smelting furnace.

Generally precious metals Au, Ag, Pt, Pd, Rh and Ir are in the world produced by using various different manufacturing methods. Gold is produced either directly, by making use of the special features of gold, or as a side product in traditional copper production. The majority of world's platinum and a remarkable share of palladium is produced by using primary electric furnaces. The majority of world's palladium production is mainly based on nickel side production from ores by means of the suspension technique, where the obtained intermediate product is nickel concentrate. An essential step in both processes is the use of a converter as part of the process. However, the use of a converter in the processes is harmful, as sulfur dioxide emissions and intermediate products caused by the transportation of melts are increased. Mattes obtained in the above described ways are further treated in hydrometallurgic plants. There are several different hydrometallurgical processes for the further refining of matte obtained from smelting, when precious metals should be recovered as side products.

The Finnish patent application 890 395 describes a manufacturing method and arrangement for producing high-grade nickel matte. According to said method, high-grade nickel matte is directly produced in a suspension smelting furnace. At least the slag from the suspension smelting furnace is reduced in an electric furnace, where the electric furnace slag and the metallicized matte are formed, so that at least part of the metallicized matte is returned as feed to the suspension smelting furnace.

From the Finnish patent 94,538, there is known a method for manufacturing high-grade nickel matte and highly oxidized slag in a flash smelting furnace, and for reducing the slag from the flash smelting furnace and for sulfurizing the created matte in an electric furnace. The matte created in the flash smelting furnace and in the electric furnace are both conducted directly to hydrometallurgical further processing. A specific object of the invention is precisely to simplify the manufacturing process of high-grade nickel matte and to avoid the use of a converter in the process.

The object of the present invention is to bring forth a new type of method for refining precious metal concentrate, so that the precious metals are advantageously recovered by making use of the suspension smelting process. Another object of the invention is to realize a refining process for a concentrate, the value of which lies in the precious metals contained therein, and where the nickel and/or copper only represent a side product in value.

The invention is characterized by what is set forth in the characterizing part of claim 1. Other embodiments of the invention are characterized by what is set forth in the rest of the claims.

The method according to the invention for refining precious metal concentrate has several advantages. The invention relates to a method for refining precious metal concentrate, and according to said method, at least the treated precious metal concentrate, the reaction gas, the slag forming agent, i.e. flux, and the flue dust are together fed into the reaction shaft of a suspension smelting furnace, so that in the suspension smelting furnace, there are created separate phases, matte and slag. The slag created in the suspension smelting furnace is conducted to an electric furnace, where metallicized matte and waste slag are formed, whereafter the matte from the suspension smelting furnace is conducted to hydrometallurgical treatment, and the slag conducted into the electric furnace is processed together with a reducing agent and possibly an agent that lowers the melting point or improves fluidity, and the created metallicized matte is conducted either to hydrometallurgical treatment or back into the suspension smelting furnace. According to the invention, in the refining of a precious metal concentrate containing precious metals, particularly platinum and palladium, there is advantageously used a suspension smelting furnace, such as a flash smelting furnace.

The method according to the invention for refining precious metal concentrate can also be utilized so that part of the supplied precious metal concentrate is replaced by sulfide concentrate. However, the process according to the invention essentially differs from said publications (FI patent 890,395 and FI patent 94538), because the raw material used in the process is precious metal concentrate and not nickel concentrate, wherefore high-grade nickel matte is not created.

According to a preferred embodiment of the invention, the matte obtained from a suspension smelting furnace and the metallicized matte obtained from an electric furnace are granulated prior to the hydrometallurgical treatment. According to various different applications of the invention, the matte from a suspension smelting furnace and the metallicized matte from an electric furnace are processed either in the same hydro metallurgical process or in different processes. According to a preferred embodiment of the invention, in the hydrometallurgical treatment, also the matte from the suspension smelting furnace is leached at least in one step. Thus the desired components of the concentrate are recovered. According to an embodiment of to the invention, also the metallicized matte from the electric furnace is leached at least in one step in the hydrometallurgical process. According to a preferred embodiment of the invention, the leaching of the matte takes place in sulfate atmosphere. According to another embodiment of the invention, the leaching takes place in chloride atmosphere. According to yet another embodiment of the invention, precious metals are recovered from the leach residue. According to a preferred embodiment of the invention, the ferrous precipitate created in the hydrometallurgical treatment of matte and metallicized matte is conducted to a suspension smelting furnace.

In the process according to the invention, the energy contained by the raw material, such as the oxidizing heat contained by iron and sulfur, is utilized more efficiently than if the concentrate were treated in a primary electric furnace. Because in the process the matte phase is separated from slag in two steps, both in the suspension smelting furnace and in the electric furnace, the recovery of precious metals is remarkably increased when compared to processing in a primary electric furnace. In the process according to the invention, the amount of created exhaust gases is remarkably smaller than when using only a primary electric furnace in the treatment of the concentrate. Along with the method according to the invention, also dust losses are reduced. A relatively smaller quantity of gas facilitates the recovery and manufacturing of sulfur dioxide either into pure sulfur dioxide or into sulfuric acid. Now the necessary investments related to the gas and sulfur dioxide are smaller than in a process based on a corresponding primary electric furnace fulfilling the ecological requirements. The elimination of the use of converters results in the same advantages described above as the fact that primary electric furnaces are not used anymore.

The invention is described in more detail below with reference to the appended drawing.

FIG. 1 Description of the process according to the invention.

FIG. 1 illustrates a suspension smelting furnace 1 to be used in the method according to the invention, such as a flash smelting furnace. In the top part of the furnace reaction shaft 3 there is fed precious metal concentrate 9, oxidizing reaction gas 10, slag-forming agent, i.e. flux 11 and flue dust 12 obtained from the cooling of exhaust gases from the waste heat boiler 6. Into the suspension smelting furnace 1, there can also be fed iron precipitate created in the hydrometallurgic units 15 and 16 in the treatment of the matte. The ingredients fed into the reaction shaft 3 react with each other, and on the bottom of the settler 4, there is formed a matte layer 8 and on top of it a slag layer 7. The gases created in the suspension smelting furnace are removed through the uptake shaft 5 into a waste heat boiler 6, wherefrom the created flue dusts 12 are recirculated back into the suspension smelting furnace, and the exhaust gases 18 are conducted for further processing. A remarkable share of the concentrate 9 are precious metals that are accumulated in the settler, mainly in the matte phase 8. The matte 8 is subjected to granulation 17, and it is conducted into hydrometallurgical further processing 15, where the matte is leached, in which case the precious metals are leached last.

The slag 7 created in the suspension smelting furnace is conducted into an electric furnace 2, in which, apart from oxidized slag and reducing agent, there also is fed, if necessary, a sulfurous or other material for lowering the melting point or for improving the fluidity in order to adjust the melting point of the matte to be created. In the electric furnace, as a result from the reduction process, there is created metallicized matte 14 and slag 13. Without a sulfur addition, the sulfur content of the metallicized matte may remain very low, and respectively the melting point and viscosity may remain high. In the electric furnace, the precious metals are mainly transferred into a matte phase 14, which is further conducted, according to the invention, either to hydrometallurgical treatment 16, together with the matte from the suspension smelting furnace or separately. Another alternative is to recirculate the metallicized matte 14 or part thereof back into the suspension smelting furnace 1. Prior to the hydrometallurgical treatment 16 of the metallicized matte 14, the matte is subjected to granulation 19. The slag 13 created in the electric furnace 2 is waste slag, i.e. it is discarded. The precious metals are recovered in a hydrometallurgical process.

Both in a suspension smelting furnace and in an electric furnace, precious metals are mainly trasferred to the matte phase, from which they are recovered in a hydrometallurgical process. Both the matte 8 from the suspension smelting furnace and the metallicized matte 14 from the electric furnace are leached either in the same leaching line or separately. The leaching steps are dependent on the contents of the precious metal concentrate to be treated. According to a preferred embodiment of the invention, the leaching is carried out in a sulfate atmosphere, i.e. the solution at some stage contains sulfate. Now the cobalt and nickel possibly contained in the concentrate are leached in the first selective pressure leaching step as sulfate. In the same step there also is leached iron that can at the same time be precipitated as iron hydroxide. Nickel is recovered as salt, or it is turned into metal in the electrolysis. In the second leaching step, copper is leached as copper sulfate, which can be separated as such or turned into metallic copper in the electrolysis. Copper sulfate can also be crystallized and fed back into the suspension smelting furnace after drying. By adjusting the degree of oxidation in the leaching process and the oxidation in the pyrometallurgical process, the sulfate balance of the solution can be affected. The precious metals are left in the leach residue. The precious metal content of the leach residue is increased for instance by means of a strong sulfuric acid and sulfur dioxide treatment. The created concentrated precipitate is a good raw material for various precious metal refineries. According to an embodiment of the present invention, the leaching is carried out in a chloride atmosphere, in which case there is used chloride gas in the leaching, and in the solution, there are created cobalt, nickel, copper and iron chlorides.

The invention is illustrated with reference to an example below.

EXAMPLE

The method according to the invention was applied for the precious metal concentrate mentioned above, so that part of said concentrate was replaced by nickel concentrate. The iron precipitate created in the hydrometallurgic unit was recirculated back into the suspension smelting furnace. The abbreviation PGM means precious metals.

The analyses and the material flow into the suspension smelting furnace:

Precious metal concentrate Nickel concentrate Fe precipitate Share % 75 22 3 Analysis Ni % 2 9 Cu % 10 3 Fe % 23 39 58 S % 20 27 SiO₂ % 28 14 Al₂O₃ % 4 1 MgO % 8 6 PGM ppm 75 3

When applying oxygen enrichment in the feed gas and a suitable degree of oxidation, as well as when taking into account the exhaust air, the oil demand for the heat balance, the recirculating dust quantities, the required fluxes and, as regards the electric furnace, the need for coke and a small amount of concentrate for the sulfurizing the matte, the following products were obtained from the suspension smelting furnace and from the electric furnace.

Material flows of the feed mixture quantity and analyses: Suspension smelting furnace Electric furnace Matte Slag Matte Slag Material flow % of feed 12 71 4 67 Analyses Ni % 20 1.3 24 0.1 Cu % 54 2.4 31 0.7 S % 21 0.2 8.0 0.3 Fe % 3.0 36 34 37 SiO % 0.0 32 0.0 35 MgO % 0.0 9.7 10 3.5 PGM ppm 440 2.2 32 0.4

The gas created in a suspension smelting furnace contains more than 10% sulfur dioxide, and is thus suitable for the production of sulfuric acid. The exhaust gases from the electric furnace are nearly free of sulfur dioxide, and consequently do not strain the environment. The method described above also functions without nickel and even so that a large part of the copper is replaced by iron, if the source material does not contain a sufficient amount of copper.

For a man skilled in the art, it is obvious that the different embodiments of the invention are not restricted to the examples given above, but may vary within the scope of the appended claims. 

1. A method for refining precious metal concentrate, comprising: a) feeding at least precious metal concentrate, reaction gas, flux and flue dust to be treated together into the reaction shaft of a suspension smelting furnace; b) creating in the suspension smelting furnace, separate phases of matte and slag; c) conducting the slag created in the suspension smelting furnace into an electric furnace, so that metallicized matte and waste slag is created d) conducting the matte from the suspension smelting furnace to hydrometallurgical treatment, and e) treating the slag conducted into the electric furnace together with a reducing agent and possibly together with a material lowering the melting point or improving the fluidity, and conducting the created metallicized matte either to hydrometallurgical treatment or back into the suspension smelting furnace.
 2. A method according to claim 1, further comprising replacing part of the precious metal concentrate to be fed into the suspension smelting furnace with sulfide concentrate.
 3. A method according to claim 1 further comprising granulating the matte from the suspension smelting furnace and the metallicized matte from the electric furnace before the hydrometallurgical treatment.
 4. A method according to claim 1, further comprising treating the matte from the suspension smelting furnace and the metallicized matte from the electric furnace in the same hydrometallurgical process.
 5. A method according to claim 1, further comprising treating the matte from the suspension smelting furnace and the metallicized matte from the electric furnace in separate hydrometallurgical processes.
 6. A method according to claim 1, further comprising leaching in the hydrometallurgical treatment, the matte from the suspension smelting furnace in at least one step.
 7. A method according to claim 1, further comprising leaching in the hydrometallurgical treatment, the metallicized matte from the electric furnace in at least one step.
 8. A method according to claim 6 further comprising carrying out the leaching in sulfate atmosphere.
 9. A method according to claim 6 further comprising carrying out the leaching in a chloride atmosphere.
 10. A method according to claim 6, further comprising recovering the precious metals are from the leach residue.
 11. A method according to claim 1, further comprising conducting the ferrous precipitate created in the hydrometallurgical treatment of matte and metallicized matte into a suspension smelting furnace. 